The present invention relates to the generation of acoustic waves and more particularly, the invention relates to the reproducible production of high frequency noise bursts.
The generation of acoustic signals has become increasingly important for the detection of flaws and defects in structural parts. Ultrasonic signals and particularly pulses are caused to traverse the material, and the interaction thereof with the acoustic waves is used to obtain information on the uniformity of texture (or lack of it) of the material. It is apparent that the resolution of this detection method depends on the wave length of the acoustic signal. Thus, it is desirable to operate with as high a frequency as possible to permit the detection of minute cracks and fissures.
Another field of applying acoustics to flaw detection relates to acoustic emissions. These are spontaneous, high frequency acoustic waves emitted upon relief of localized tension and stress in the structural part. The emission of these waves can be used as a criterion to indicate the internal development of a crack, fissure or the like. It is inevitable that during the continuous monitoring of acoustic emissions in a structure noise is also picked up from non-relevant sources. This noise can be similar in many ways to the acoustic emissions generated by a growing crack, and it is a particularly vexing problem to identify an acoustic emission and separate it from the noise. One characteristic which is used to accomplish this is their frequency spectrum. Identification and interpretation of acoustic emissions from their spectrum relate directly to the calibration of the monitoring pickup transducers and of the various wave propagation paths in the structure under test. It was found that the existing devices for producing bursts of acoustic energy for purposes of calibration are either too narrow banded or afford little control over their frequency spectrum.